1. Field of the Invention
The present invention relates to a positioning system and a positioning method for estimating the position of a receiver having a wireless communication function.
2. Related Background Art
There has been proposed in the past a technique for estimating the position of a cellular terminal (mobile communication terminal) or other such receiver on the basis of the reception strength (reception level) of the radio wave received by the receiver. Japanese Patent Application No. H7-231473, which is a Japanese laid-open patent application, discusses a technique for estimating the position of a receiver by taking information indicating the relation between the reception strength of a radio wave received from a specific base station and the position at which this radio wave was received, storing this information in a database, and using this database to match with reception strength.
Known in the past was a technique for using a GPS (global positioning system) to find a receiver. Another known method is AGPS (assisted GPS), in which positioning is performed by utilizing assist data indicating a satellite position, etc., acquired from a mobile unit communication network. If finding a position by GPS should fail (that is, if the GPS positioning result is not fixed), base station positioning, in which a signal from a mobile unit communication network is used to perform positioning, or positioning called hybrid positioning, which combines a signal from a GPS satellite and a signal from a mobile unit communication network, is sometimes performed.
With conventional methods, however, whether or not positioning by GPS was successful could not be decided without actually commencing the positioning. Also, GPS positioning that entailed the above-mentioned decision took a long time. And a UE-based AGPS positioning method, in which computation is performed by the receiver, took an especially long time. For example, indoors where a signal cannot be received from a GPS satellite, after GPS positioning has proven impossible, the system switched to a different type of positioning such as base station positioning, so the overall positioning took a long time. In this case, the positioning precision is poor for how long the positioning takes. Also, in cities and the like, even outdoors the effect of multipulses can prevent good GPS positioning precision from being attained.